Fast Neutrons
Neutrons with high kinetic energy above 1 MeV, produced directly in fission without moderation.
Fast neutrons emerge from fission with average energy ~2 MeV, possessing low fission cross-sections in U-235 but enabling breeding in Pu-239. Fast reactors like sodium-cooled designs use them to minimize moderation and transmute U-238. They cause more material damage due to higher displacement per atom.
Why it matters now
Fast neutrons underpin advanced reactors like Natrium and ARC-100 pursuing 2026 demos, addressing HALEU bottlenecks by breeding fuel amid 2030s fuel shortages.
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